The disclosures of U.S. Pat. No. 6,137,112 to McIntyre et al., U.S. Pat. No. 5,691,537 to Chen et al., and U.S. Pat. No. 4,667,111 to Glavish et al. are hereby incorporated by reference as if fully set forth herein.
The present invention relates generally to time delay measurements and more particularly to apparatus and methods for measuring time delays between signals and time of flight energy measurement systems for use in ion implantation systems.
Measurement of time delays are needed or desired in a variety of situations, such as for measuring the average kinetic energy of ion beam ions in ion implantation systems for ion doping semiconductor workpieces, time of flight (TOF) measurements in particle accelerators, signal processing, echo sounding, etc., wherein there is a need for precise determination of the time delay between two pulse streams, such as those derived from a common frequency (clock). Some conventional systems trigger on a signal waveform to obtain a timing reference (time stamping) for real-time waveform display or data recording (e.g., scope, data acquisition). The timing accuracy (e.g., time jitter) in such approaches is strongly dependent on the rise-time (e.g., slope, or waveform) of the pulse, and deteriorates rapidly with decreasing signal-to-noise ratio (SNR) (e.g., small signal amplitudes).
In ion implantation systems or ion implanters, time delay measurements are used in time of flight energy measurement systems for measuring ion beam energy. Any inaccuracies in the time delay measurement in such systems leads to inaccuracies in determining the actual energy of the ion beam being used to implant semiconductor wafers or other workpieces. Accordingly, there is a need for improved time delay measurement systems and methods for determining a time delay between two input signals and improved energy measurement systems for ion implanters by which better time delay and energy measurements can be achieved.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention is directed to methods and apparatus for measuring time delays between pulse streams or other input signals, which may be employed in any number of applications in which time delay measurements are desired, including but not limited to ion implantation systems. One aspect of the invention relates to time delay measurement systems which may be employed in ion implantation systems for determining ion beam energy, particle accelerators, signal processing, echo sounding, or other applications. A time delay measurement system is provided, comprising first and second input channels receiving first and second input signals, wherein the input signals may be of any character, for example, such as electrical pulse signals or pulse streams obtained from time of flight (TOF) system probes in an ion implanter. In such an example, the pulses may represent passage of groups or bunches of ions through or past the TOF system probes along an ion beam path.
A delay apparatus is associated with one of the channels, which provides a variable delay to a corresponding one of the input signals, and a correlator apparatus is provided having first and second correlator inputs receiving outputs from the first and second channels, respectively. The correlator apparatus provides a correlator output signal having an ascertainable correlator output value, such as a local minima, local maxima, or other discernable signal value, when the outputs from the first and second channels are substantially aligned in time. The variable delay may be manually or automatically adjusted by measurement circuitry (e.g., such as a time of flight energy measurement circuit in an ion implantation system), while the correlator output is monitored to determine a delay value corresponding to the ascertainable correlator output value when the outputs from the first and second channels are substantially aligned in time. For instance, the measurement circuitry may sweep the variable delay value and search for a relative minima or other ascertainable value at the correlator output. At this point, the time delay between the input signals is determined to be the delay value at which the ascertainable correlator output value is reached.
In another aspect of the invention, the time delay measurement system further comprises amplitude adjustment apparatus, which operates to selectively adjust a gain associated with one of the channels, wherein the measurement circuitry may operate to selectively control the delay apparatus and the amplitude adjustment apparatus. This feature allows the variable delay value to be initially adjusted or swept (e.g., while holding the amplitude constant) to determine a first delay value corresponding to the ascertainable correlator output value when the channel outputs are substantially aligned in time. Thereafter, the delay is held at this first delay value while a gain of one of the channels is swept or otherwise adjusted by the amplitude adjustment apparatus while the correlator output signal is monitored. A first gain value is thus determined corresponding to the ascertainable correlator output value, after which another delay sweep may be performed to identify a refined delay value. This technique of alternatively adjusting the variable delay and the variable amplitude may be repeated any number of times to refine the time delay estimate using the correlator apparatus to indicate substantial or exact temporal alignment of the first and second channel outputs.
In yet another aspect of the invention, the time delay measurement system may further comprise an error correction circuit disposed between input signals and the first and second channels. During calibration, the error correction circuit provides one of the input signals to both channels having substantially equal amplitudes and no relative time delay therebetween.
Still further aspects of the invention provide ion implantation systems and time of flight ion beam energy measurement systems therefor, for measuring an average kinetic energy of an ion included in a selected ion pulse of an ion beam. The time of flight ion beam energy measurement system comprises first and second sensors spaced apart from one another by a sensor distance along an ion beam path. The second sensor is located downstream of the first sensor along the path, wherein the first sensor generates a first sensor signal when an ion pulse of the ion beam passes the first sensor and the second sensor generates a second sensor signal when the ion pulse passes the second sensor. The time of flight system further comprises a time delay measurement system for measuring a time delay between the first and second sensor signals, comprising a delay apparatus and a correlator apparatus such as those described above.
The time of flight system also comprises measurement circuitry receiving the correlator output signal, the measurement circuitry being adapted to control the delay apparatus to selectively adjust the variable delay while monitoring the correlator output signal, to determine a first delay value corresponding to the ascertainable correlator output value when the outputs from the first and second channels are substantially aligned in time. A measured ion beam energy may then be determined by the measurement circuitry according to the first delay value, a mass of particles in the ion beam, and the sensor spacing distance.
The time delay measurement system may further comprise amplitude adjustment apparatus allowing sweeping or adjustment of a gain associated with one of the channels. In one example, the measurement circuitry operates to set the variable delay to the first delay value, selectively adjust the gain while monitoring the correlator output signal and determine a first gain value corresponding to the ascertainable correlator output value, set the gain to the first gain value, and again adjust the variable delay while monitoring the correlator output signal. The measurement circuitry may then determine a second delay value corresponding to the ascertainable correlator output value when the outputs from the first and second channels are substantially aligned in time, and determine a measured ion beam energy according to the second delay value, the mass of particles in the ion beam, and the sensor distance.
Another aspect of the invention involves methods for measuring a time delay between first and second input signals, which may be employed in measurement of time delays in ion implantation systems, particle accelerators, test equipment, signal processing, echo sounding, or other applications wherein measurement of time delays is desired. The methods comprise providing a variable time delay in one of the input signals, correlating the input signals, such as through subtraction, to generate a correlated output signal, adjusting the variable time delay to a value at which the correlated output signal is an ascertainable value when the input signals are substantially aligned in time, and determining a measured time delay according to the delay value at which the correlated output signal is the ascertainable value. The methods may further comprise adjusting a variable amplitude in one of the input signals to a value at which the correlated output signal is a minimum, and again adjusting the variable time delay to a delay value at which the correlated output signal is at the ascertainable value before determining the measured time delay.
Still another aspect of the invention provides methods for measuring an average kinetic energy of particles in an ion beam. The method comprises inputting first and second input signals from spaced ion beam sensors in an ion implantation system, providing a variable time delay in one of the input signals, and correlating the signals to generate a correlated output signal. The method further comprises adjusting the variable time delay to a delay value at which the correlated output signal is an ascertainable value when the input signals are substantially aligned in time, and determining a measured beam velocity according to the delay value at which the correlated output signal is the ascertainable value and according to a spacing distance between the sensors. An average kinetic energy of particles in the ion beam is then computed according to the measured beam velocity and a mass of particles in the ion beam. The method may further comprise adjusting a variable amplitude in one of the input signals to a value at which the correlated output signal is the ascertainable value, and again adjusting the variable time delay to a delay value at which the correlated output signal is the ascertainable value when the input signals are temporally aligned before determining the measured time delay.
Yet another aspect of the invention provides a method for calibrating a time of flight energy measurement system in an ion implantation system, the method comprising providing a DC ion beam at a known DC beam energy along a beam path in the ion implantation system, modulating the DC ion beam with a small AC component, measuring a beam energy using the time of flight energy measurement system, and calibrating the time of flight energy measurement system according to the measured beam energy and the known DC beam energy. Modulation of the DC ion beam may be accomplished, for example, by energizing an RF accelerator in the ion implantation system at a low voltage to generate pulse signals in time of flight system probes without substantially changing the average energy of the ion beam. Calibrating the time of flight energy measurement system may comprise, for example, computing an energy offset value according to the measured beam energy and the known DC beam energy.
To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.